20 



BOTANY OF CROP PLANTS 



weeks. New hairs are constantly formed anew at the an- 

 terior end of the root-hair zone, while those at the posterior 

 end are dying. Root hairs do not become roots. 



Structure of a Root Hair.^The root hair is a single cell. 

 It is a simple, lateral prolongation of a border cell of the cor- 

 tex (Fig. 9). It has the shape of a slender tube which may, 

 however, become greatly contorted in its growth between and 

 about soil particles. Root hairs vary in length from a frac- 

 tion of a millimeter to 7 or 8 millimeters. The walls are thin 





^^^^^S^^^^S^^S^'f 



Fig. 9. — Root hairs. (AfterGager.) 



and of almost pure cellulose. A thin layer of protoplasm 

 ma-y line the walls, and the nucleus usually occupies a posi- 

 tion near the apex. The central vacuole is large, and is filled 

 with cell sap. The cell sap contains water, and various or- 

 ganic and inorganic substances in solution. 



Effect of External Factors upon Development of Root 

 Hairs. — Most air and water roots have no root hairs. Soil 

 roots, such as those of conifers, oaks, and others that are 

 surrounded by a fungus (mycorrhizal growth) possess no root 

 hairs. In the case of ordinary soil roots, root -hair develop- 

 ment is usually meager in very wet soil. Corn roots develop 

 root hairs in abundance in moist air, but none at all in water. 

 The absence of root hairs in very wet soil, and in water, is 

 probably to be attributed to poor oxygen supply. In a 



